Traumatic brain injury (TBI) continues to be one of the most common causes of death and morbidity in persons under age 45, even in western societies. A reported 1.7 million people suffer from TBI annually in the United States alone, resulting in an estimated per annum total cost of over $60 billion. Historically, prevention of skull and brain injury has focused on the use of helmets as external cranial protection. This approach is fundamentally flawed as helmets have provided benefit for only major penetrating brain injuries and skull fractures. These occur in a very small fraction of head injuries in civilian sphere. Military statistics have shown that even on the battlefield, less than 0.5% of TBI is from a penetrating object. However, both military personnel and athletes are subjected to high velocity acceleration-deceleration mechanisms that are not mitigated by helmets and lead to concussive injury to the brain. In large part, the human brain's relative freedom of movement within the cranial cavity predisposes to both linear and rotational force vectors, with resultant energy absorption resulting in cellular disruption and dysfunction, sometimes with delayed cell death.
The skull and spinal canal contain only nervous tissue, connective tissue and fat cells and their interstitium, blood, and cerebrospinal fluid (CSF). The non-fluid contents do not completely fill the rigid container delimited by the skull and bony spinal canal, leaving a ‘reserve volume’ that is occupied by the fluid components. The change in volume inside a container for a given change in pressure is termed ‘compliance’. Increases in volume of the contents of the skull and bony spinal canal, within the range of reserve volume, occur at low container pressures (due to the high compliance of the system). Acceleration or deceleration of the skull can result in a differential acceleration or deceleration between the skull and its contents when the brain and fluids collide with the inside of the skull. TBI may occur because of compression, stretching, or tearing of tissue and blood vessels as a result of the brain impacting the skull. Considering the semi-solid properties of the mammalian brain, this effect is referred to as “SLOSH”.
While helmets are effective in preventing the infrequent penetration or fracture of the skull, they have little ability to limit SLOSH effects. Mitigating SLOSH by increasing the pressure of the fluid contents of the brain can significantly reduce the propensity for damage to the brain tissue or its blood vessels by reducing the compressibility of the brain. The reduction in compressibility results in reduced absorption of kinetic, acoustic, thermal, and vibrational energy by the brain.
The same concussive events that produce TBI can also have damaging effects to the inner ear, spinal cord and structures of the eye. Sensory neural hearing loss is noted to occur at a rate of 85% in TBI. Concurrent injuries to the auditory system as a result of acute blast trauma and resultant traumatic brain injury accounted for one-quarter of all injuries among marines during Operation Iraqi Freedom through 2004—the most common single injury type. Auditory dysfunction has become the most prevalent individual military service-connected disability, with compensation totaling more than $1 billion annually.
Although one might expect blast waves to cause tympanic membrane rupture and ossicular disruption (thus resulting in conductive hearing loss), available audiology reports showed that pure sensory neural loss was the most prevalent type of hearing loss in patients. An observational study performed from 1999-2006 found that 58 percent of active-duty soldiers who complained of hearing loss were diagnosed with pure sensorineural loss. Data from this study revealed that 38 percent of the patients with blast related TBI also reported sensory neural tinnitus (ringing in the ears).
The sites for sensory neural hearing loss are the inner ear structures referred to as the cochlea and vestibular apparatus (semicircular canals). Both of these structures are fluid filled and therefore susceptible to SLOSH induced energy absorption. The tympanic and vestibular canals of the cochlea are also fluid filled and transmit pressure and fluid waves to the delicate hair cells of the organ of corti. The auditory hair cells react directly to the vibrations in the liquid in which they are immersed rather than to transverse vibrations in the cochlear duct. The cochlea and its associated hair cells are particularly susceptible to SLOSH energy absorption.
Approximately 30 ml (21%) of a total CSF volume of 140 ml resides within the spinal axis, and about one-third of the compliance of the CSF system has been attributed to the spinal compartment. As in the brain, increasing the pressure of the CSF within the spinal compartment reduces the susceptibility of the spinal compartment to concussive injuries by increasing the elasticity of the contents of the spinal column, thereby reducing the amount of energy absorbed by the contents of the spinal column when subjected to a concussive force.
Of 207 severe eye injuries in a report of military casualties in Operation Iraq Freedom OIF, 82 percent were caused by blast and blast fragmentation. Eye injuries accounted for 13 percent (19/149) of all battlefield injuries seen at a combat support hospital during Operations Desert Shield and Desert Storm. Hyphema (blood within the anterior chamber) and traumatic cataract were the most common findings in closed globe injuries, the majority (67%) of eyes sustained orbital injury. Of the service members experiencing combat ocular trauma (COT) in Operation Enduring Freedom, 66 percent also had TBI. Simply stated, roughly two-thirds of the combat related eye injuries were closed blast wave energy absorptions resulting in rupture.
Traumatic brain injury, or the concussive or blast-related events leading to TBI, has also been found to be a leading cause of anosmia (loss or impairment of olfactory function, i.e., sense of smell). Certain studies have reported that a large proportion of patients with post-traumatic anosmia exhibit abnormalities in the olfactory bulbs and in the inferior frontal lobes, suggesting in the latter case that reducing TBI can reduce the risk of anosmia. While loss or impairment of olfactory function can be more than a nuisance to humans, the same injury to Breecher dogs (e.g., bomb sniffers) can be catastrophic. Breecher dogs are inherently exposed to the risk of concussive events and their primary purpose is to help soldiers avoid such an event. Preventing or reducing the likelihood of TBI and associated loss of smell can be critical to the Breecher dog's mission.
Standard prophylactic measures designed to protect the brain against injury in the case of head trauma have hitherto included only various helmets. Helmets are primarily designed to protect the skull from penetrating injuries and fractures, but less so from pathological movements of the brain, exemplified by the classic cerebral concussion. Moreover, helmets have no meaningful effect on blast-related injuries to the ear, spinal column and eyes.